If you test an unknown substance with the biuret reagent and it turns purple, what is present?

The Biuret Test is done to show the presence of peptide bonds, which are the basis for the formation of proteins. These bonds will make the blue Biuret reagent turn purple. 

Process

• add an equal amount of NaOH to a solution of the food, mix carefully.
• add a few drops of 1% CuSO4, do not shake the mixture.
• a PURPLE/MAUVE COLOR is a positive result: protein is present.

Explanation

• The reagent used in the Biuret Test is a solution of copper sulfate (CuSO4) and sodium hydroxide (NaOH).
• The NaOH is there to raise the pH of the solution to alkaline levels; the crucial component is the copper II ion (Cu2+) from the CuSO4.
• When peptide bonds are present in this alkaline solution, the Cu2+ions will form a coordination complex with 4 nitrogen atoms from peptide bonds.

• The complex of Cu2+ ions and nitrogen atoms make the color of CuSO4 solution changes from blue to violet.
• This color change is dependent on the number of peptide bonds in the solution, so the more protein, the more intense the change. 

This post on blogspot.com

By Eric Bank, MBA, MS Finance

The biuret test is a chemical assay that detects the presence of proteins in a sample. The test relies on a color change to confirm the presence of proteins. If proteins are found, the sample will turn violet. It’s a funny thing, but the biuret test doesn't involve the chemical biuret, which is derived from urea. Biuret isn't a protein, but it gives a positive result to the biuret test.

The biuret test uses an alkaline mixture, or reagent, composed of potassium hydroxide and copper sulfate. The normal color of biuret reagent is blue. The reagent turns violet in the presence of peptide bonds -- the chemical bonds that hold amino acids together. The proteins detected must have at least three amino acids, which means that the protein must have at least two peptide bonds. The reagent’s copper ions, with a charge of +2, are reduced to a charge of +1 in the presence of peptide bonds, causing the color change. The techniques of absorption spectroscopy, which identify the electromagnetic frequencies a sample will absorb, allow testers to quantify the concentration of protein in a sample.

In practice, testers mix equal amounts of potassium hydroxide and the sample material in a test tube. If the test material is solid, like a food, it’s first liquefied or dissolved. The tester then adds a few drops of a 1 percent aqueous solution of copper sulfate, being careful not to shake the mixture. A violet color indicates protein in the sample. Scientists measure the amount of protein by taking a spectroscopic reading at a wavelength of 540 nanometers (nm).

Scientists have found ways to modify the biuret test and increase its sensitivity. The Smith assay increases sensitivity a hundredfold. It uses bicinchoninic acid as the source of copper and turns purple when protein is present. A spectroscopic reading at 562 nm reveals the amount of protein in the sample. The Lowry protein assay uses the phosphate salts of the elements molybdenum and tungsten. The absorption at 750 nm wavelength light in a spectroscope indicates the concentration of protein present.

Testers expect to find some protein in human urine. However, if the level repeatedly exceeds the normal limit, it can indicate problems with the kidneys. Other possible causes include diabetes, heart disease, high blood pressure, leukemia, lupus and pregnancy. Excessive protein levels in your blood can indicate dehydration, chronic inflammation or infection. Conditions that are more serious include HIV/AIDS, multiple myeloma, abnormal bone marrow conditions and amyloidosis. A high-protein diet is not a cause of high protein levels in the bloodstream or urine.

In order to continue enjoying our site, we ask that you confirm your identity as a human. Thank you very much for your cooperation.